Sedentary behaviors, activities performed while sitting and that involve low levels of energy expenditure (e.g., watching TV, working on a computer, etc.), pose a distinct and significant health risk. Engagement in higher levels of sedentary behavior-independent of physical activity-is associated with several adverse health outcomes, and is an independent risk factor for cardiovascular-related and all-cause mortality. Obese individuals are at particular risk for engagement in high levels of sedentary behavior. Bariatric surgery patients, in particular, spend 80% of their objectively-assessed waking hours being sedentary, considerably higher than the percentage of time sedentary reported in the general population. In the K award, we attempted to intervene on bariatric surgery patients' high levels of sedentary behavior. Participants received 6 weeks of individual instruction in strategies (self-monitoring, goal setting, etc.) to reduce sedentary time and feedback on objectively-monitored sedentary time at the end of this period. However, this approach did not change sedentary time and participants reported difficulty in monitoring their SB. Additionally, the efficacy of this approach may have been limited by the lack of immediate feedback on SB. The proposed R03 project incorporates lessons learned from research supported in the K01 (DK083438-03) by employing a mobile health (mHealth) approach that utilizes widely adopted smartphone technology to intervene on sedentary behaviors in bariatric surgery patients. This new approach addresses limitations of the previous approach via unique capabilities, including: 1) automatic monitoring of patients' sedentary behaviors across all contexts in real time in their natural environment, and 2) use of real-time data to provide immediate individually-tailored, goal-driven prompts and feedback to encourage patients to substitute sedentary behavior with walking activity. Additionally, persistent graphical feedback indicating progress toward sedentary goals will be presented on the smartphone in an entertaining format to encourage adherence to the intervention protocol. Two studies will be conducted to evaluate the feasibility and acceptability of the new mHealth smartphone approach. Study 1 will compare three smartphone goal-driven prompting/feedback schedules to determine which schedule produces the best adherence and has the greatest impact on sedentary time. Study 2 will use the best prompting/ feedback schedule identified in Study 1 to assess the impact of the mHealth smartphone approach on longer-term adherence and sedentary time over a 4-week period. This project addresses the public health problem of increasing sedentary time by exploring the viability of popular mobile smartphone technology for reducing sedentary behaviors in bariatric surgery patients, a highly sedentary population. If efficacious, this approach could also be made widely available to help other individuals monitor and decrease their sedentary time for very little cost using a device they already own.